Conventionally, projection type image display devices such as projectors focus intense light from a light source onto an image display element such as a liquid crystal panel or DMD (Digital Mirror Device), so as to perform modulation in accordance with an image signal, and enlarge and project an image formed on the image display element, thereby obtaining a high-resolution large screen.
In such projection type image display devices, the imaging element exposed to intense light, the light source, a device power source, etc. generate a large amount of heat, and the absence of appropriate cooling not only influences image quality, but also causes thermal breakdown. For this reason, projection type image display devices have a structure in which air is drawn into the device by a fan to cool the device interior.
However, powder dust is drawn in along with the air from outside the device, and the attachment of such powder dust to the image display device and light source periphery causes a reduction in brightness and color irregularities.
For this reason, a filter for preventing the intake of powder dust is disposed in an air intake, and in the case of a mainstream urethane filter, filter cleaning is required every several hundred hours. Also, although a urethane filter physically captures powder dust in small apertures to prevent intake, powder dust whose size falls below 10 μm cannot be blocked. The intake of microparticles such as yellow sand and tobacco smoke therefore cannot be prevented.
On the other hand, in an electrostatic filter, even small-sized dust can be captured since powder dust is trapped by static electricity, and a large loss in pressure is suppressed since the apertures are large. However, when the capturing capability is exceeded, the effect of the static electricity no longer can be obtained, and powder dust enters the device interior instead of being captured. Also, an electrostatic filter cannot be cleaned, and replacement with a new product is required every several hundred hours.
However, projection type image display devices are often for ceiling-mounted use, so that maintenance is difficult because of an installation condition in which filter replacement cannot be performed easily. In view of the same problem, there are known, for example, air-cleaning devices having structures such as the following.
For example, Patent Document 1 discloses an ozone deodorization device having a structure in which an electrostatic filter is formed into a long belt and windably provided between two rotating rollers, and the electrostatic filter is wound automatically by a motor according to the operating time of the device. This structure enables using an electrostatic filter for a long time without replacement with a new product. The application of this device to a projector is considered to be able to extend the filter replacement time.
Also, Patent Document 2 discloses a similar structure in an air-cleaning device, in which an unused-filter storage unit and a used-filter storage unit are integrated in a cassette that can be opened with the provision of a hinge.    Patent Document 1: JP H5-49830A    Patent Document 2: JP H6-269696A